(1) Field of the Invention
The present invention generally relates to a suspension system, and more particularly to an apparatus for controlling the damping force of a shock absorber provided in a suspension system of an automotive vehicle on the basis of the condition of a road surface.
(2) Description of the Prior Arts
There is previously known an apparatus for controlling the damping force of a shock absorber in order to improve riding comfort obtained when the vehicle is traveling. Such an apparatus detects the condition of a road surface on which the vehicle is traveling, and alters the setting of the damping force from a high level (hard state) to a low level (soft state) or vice versa on the basis of a roughness of the road surface.
A conventional damping force control apparatus uses an ultrasonic sensor which senses a roughness of the road surface. Another conventional damping force control apparatus disclosed in Japanese Laid-Open patent application No. 62-221907 judges whether or not the road surface is rough by detecting the expanding/contracting acceleration of the shock absorber. Still another conventional damping force control apparatus disclosed in Japanese Laid-Open patent application No. 63-6238 judges whether or not the road surface is rough by detecting the damping force of the shock absorber.
In conventional damping force control apparatus as described above, when a predetermined condition for altering the setting of the damping force is satisfied, the setting of the damping force of the shock absorber is altered at a maximum operating speed of a damping force alteration mechanism in order to rapidly alter the setting of the damping force. Thus, when the setting of the damping force is altered while the damping force is maintained in the high (hard) level, the damping force is rapidly changed, so that a shock due to the rapid switching of the damping force occurs. The occurrence of such a shock causes the driver or passengers to feel uncomfortable.
In order to eliminate the above-mentioned problem, it may be possible to alter the setting of the damping force at a decreased speed. However, the response characteristic of the shock absorber will deteriorate if the setting of the damping force is simply altered at a low speed. For example, the setting of the damping force will not be altered rapidly in such a case that no shock substantially takes place (that is, in the case where the damping force is maintained at the low (soft) level).